This invention relates to a method to recover and purify aromatic diacids.
Terephthalic acid is typically produced by oxidation of paraxylenes and then separation of the oxidized paraxylenes by precipitation, and then washing the precipitate to remove trace solvents.
In a process that is not currently commercially utilized, it is known to isomerize aromatic carboxylic acids to terephthalic acids utilizing a solid state isomerization of salts of the aromatic carboxylic acids in the presence of carbon dioxide and salts such as potassium carbonate or cadmium fluoride, as disclosed in, for example, U.S. Pat. No. 2,891,992. This process results in solid salts that still must be purified and changed from the salt to the diacid. Mineral acids in aqueous solutions have been used to acidify the salts. The dicarboxylic acids are significantly less soluble in aqueous solutions than the corresponding salts, and they therefore readily precipitate from the solution upon acidization or upon acidization and cooling. Acidification of the aromatic dicarboxylic acid salts with mineral acids results in a solvent stream that contains the salt of the mineral acid. These streams can generally not be reused in the process, and must therefore be disposed of.
A process wherein salts resulting from the acidification step could be recycled would be preferable. U.S. Pat. No. 2,905,709 suggests such a process. The process of ""709 involves providing an aqueous solution of a salt of terephthalic acid, and acidifying the solution with an aromatic acid. The aromatic acid can be readily recovered and recycled, but use of the aromatic acid has other disadvantages. Trace amounts of the aromatic acid which are in the terephthalic acid precipitate must be removed to obtain a composition useful for polymerization because the aromatic acids are chain terminators in polymerization. It would therefore be desirable to have a process wherein salts resulting from the acidification step could be recycled, but which would not introduce a compound which acts as a chain terminator.
It is therefore an object of the present invention to provide a process to acidify and precipitate carboxylic diacids in a process wherein a waste stream containing salts of a mineral acid is not produced.
This and other objects are accomplished by a method to produce dicarboxylic or tricarboxylic aromatic acid from salts of such acids, the method comprising the steps of providing an aqueous solution of a salt of a dicarboxylic or tricarboxylic aromatic acid, the aqueous solution having a pH of about 7 or greater; contacting the aqueous solution with sufficient carbon dioxide to lower the pH of the aqueous solution, resulting in precipitation of the dicarboxylic or tricarboxylic aromatic acid; separating precipitated dicarboxylic or tricarboxylic aromatic acid from the solution; and recovering carbon dioxide from the solution until the pH of the solution is at least about 7.
The aqueous solution remaining after terephthalic acid is precipitated contains salts of carboxylic acid, such as potassium carboxylate. Carbon dioxide can therefore be recovered from this solution by heating the solution. The remaining solution would be of a high pH, and could be used to generate an aqueous solution of the salt of the terephthalic acid.
It has also been found that contact of precipitated aromatic diacids which contain monosalts of the diacids can be disproportioned to disalts and diacids by contact with water. Further, when the desired diacid is not the least soluble species present (for example, when isophthalic acid is being produced and terephthalic acid is present), the less soluble species can be precipitated first, and then the more soluble species, thus providing a solid of the less soluble species.
The present invention may be utilized to recover a wide variety of dicarboxylic and tricarboxylic aromatic acids from solutions of salts of such acids. The preferred dicarboxylic and tricarboxylic acids include terephthalic acid, phthalic acid, trimesic acid, isophathalic acid, and naphthalene dicarboxylate acid. Particularly preferred are terephthalic acid and 2, 6 naphthalene dicarboxylic acid, based on their low solubilities and high commercial value. Other one or two aromatic ring compounds containing two or three acid groups, and mixtures thereof, can be prepared from salts of the acids.
The starting material of the present invention is preferably a Henkle reaction product, or the dicarboxylic acid salt of an aromatic or naphthalene that has been isomerized by contact at temperatures of above about 300xc2x0 C. with salts under carbon dioxide pressure. Such reaction products are in a salt state, and are generally subsequently converted to diacids and separated from more soluble salt byproducts. Such salts can be dissolved in water, and then contacted with carbon dioxide according to the present invention in order to acidify and separate desirable products.
The first pKa of carbonic acid is 6.35, and the second is 10.33. Thus, there is a gap of almost 3, or almost 1000 in equilibrium constant terms between the acidity of carbonic acid and the strong acid group of dicarboxylic aromatic acids. It is therefore unexpected that carbon dioxide, at reasonable partial pressures, could be useful to participate the dicarboxylic aromatic acids according to the present invention. Additionally, the carbon dioxide can be readily recycled, reducing the costs of the materials and reducing environmental discharges.
To provide sufficient carbonic acid in solution, the final temperature, during precipitation of the dicarboxylic or tricarboxylic aromatic acid is less than about 150xc2x0 C., and preferably between about 0xc2x0 C. and about 100xc2x0 C. The partial pressure of carbon dioxide is preferably between about 15 and about 1000 psi, and most preferably between about 700 and about 1000 psi, depending on the temperature, concentration of acid and the composition of the salt solution. The optimum pressure depends on the pKa of the acid to be precipitated, the concentration of the acid salt, the fraction of salt required to be converted to the acid, and other factors such as the expense of providing equipment capable of higher pressures.
The precipitate could be at least in part a monosalt, in which case the precipitate could be washed with water to disproportionate the monosalt into disalts and diacids. The disalts could then be removed by the water wash. At higher concentrations of the dicarboxylic or tricarboxylic acid salts, the precipitate will contain a significant portion of monosalts, thus the water wash and disproportionation is preferred when the concentration of the dicarboxylic or tricarboxylic acid salts are greater than about five percent.
It has also been found to be advantageous to perform two stages of contacting solutions of the dicarboxylic or tricarboxylic aromatic acid salts with carbon dioxide.
Crystals of precipitated dicarboxylic or tricarboxylic acids can be readily removed from the mother liquor by filtration, centrifugation, or gravity settling. These crystals may then be washed with water, preferably hot water, or solvents such as acetic acid to remove impurities.
The method of the present invention is most applicable to the least soluble product present in the product mixture, although more soluble products may be recovered by sequential differential precipitation as for example by raising the carbon dioxide pressure, collecting a crop of the least soluble acid, raising the carbon dioxide pressure or lowering the temperature to collect a crop of the more soluble acids, etc.
Carbon dioxide can be easily recovered from the solution by flashing off at a lower temperature, or by heating the liquid, or by a combination of the preceding. It is preferably that enough carbon dioxide be removed so that the liquid returns to a pH of greater than about 7, and preferably greater than about 9. The pH of the remaining liquid after removal of the carbon dioxide is preferably sufficiently high that the remaining liquid could be recycled to provide a basic solution for preparation of salts of the dicarboxylic acid. Carbon dioxide can be compressed, and recycled to the solution of a salt of a dicarboxylic or tricarboxylic aromatic acid to acidify the salt, and cause precipitation of the dicarboxylic or tricarboxylic acid.